Formation of solid solutions with metal oxides can open new application fields of metal oxides. That is, metal oxide solid solutions can be applied to a broader range of fields than individual metal oxides can. For example, nano-sized metal oxide solid solutions find applications in various industrial fields, such as UV light shielding, chemical reaction catalysis, optical devices, and so on.
Depending on the phase in which materials are processed, the preparation of metal oxide solid solutions can be classified into vapor, liquid and solid phase processes.
A vapor phase process, exemplified by flame combustion pyrolysis, laser vaporization, plasma vaporization and spray pyrolysis, generally involves the vaporization of metal or metal precursors, which is followed by oxidation.
U.S. Patent Publication No. 2002/018741 A1 discloses the preparation of solid solutions of titania with various metal oxides by use of flame combustion pyrolysis, stating that titania can show improved photocatalytic activity when it forms solid solutions with Al2O3, PtO2, MgO and/or ZnO. The pyrogenic process has advantages of being simple, as in the production of individual metal oxide, and being able to produce fine particles with homogeneity. However, the process suffers from disadvantages of high energy consumption, expensive facilities and low productivity.
Typical of a solid phase process are firing and mechanochemical synthesis. A firing method, which is traditionally used for the preparation of inorganic particles, comprises thermally decomposing precursors for a long period of time at a high temperature in a furnace in the presence of oxygen to produce metal oxides and crystallizing the metal oxides, followed by pulverization to fine particles. The firing method is simple, but has the disadvantage of easily allowing the incorporation of impurities into products and requiring a long period of reaction time at high temperatures.
Nature Materials, Vol. 1, 123-128 (2002) introduces a mechanochemical synthesis method of doping various metals such as Mg2+, Al3+, Ti4+, Nb5+ and/or W6+ to improve the electric conductivity of LiFePO4, a cathode material of secondary lithium batteries. Such a mechanochemical synthesis method is characterized by a mechanical stimulation (high-speed ball milling) for activating the surface of metal precursor sufficiently to induce reaction thereon. In the process of milling, however, impurities may be incorporated from vials. Additionally, the mechanochemical synthesis requires a long reaction time and a separate calcination process.
As for the liquid phase process, it includes hydrothermal synthesis techniques or sol-gel techniques and the like. U.S. Pat. No. 5,269,972 discloses a method of preparing doped zinc oxide microspheres with resort to a sol-gel technique. This method is able to produce uniform spherical micro-particles, but cannot be applied to mass production.
Hydrothermal synthesis techniques, most widely used in the liquid phase process, use water as a reaction medium or a reactant for thermal synthesis. EP 1,055,642 A2 describes the production of metal oxide-doped ceria by use of a hydrothermal synthesis technique. According to this prior art, metal ions as dopants are required to be larger in ion radius or lower in valance than Ce4+. Metal ions meeting these requirements include Ca2+, Y3+, La3+, Nd3+, Eu3+, Tb3+, Sm3+, Mg2+, Sr2+, Ba2+, and Ce3+. With high UV shielding performance, low catalytic activity, and excellent transparency, the metal oxide-doped ceria may be suitable as UV-light blocking cosmetic materials.
It is reported that ZnO-doped CeO2 exhibits excellent UV-light blocking effects, but low oxidation catalytic activity (Ruxing Li et al., Materials Chemistry and Physics, 75, 39-44 (2002)). Through the hydrothermal synthesis technique, nano-sized particles can be prepared at relatively low temperatures and pressures. The hydrothermal synthesis technique, however, bears formidable disadvantages: expensive oxidant employment, by-product production, and post-processes for treating waste acids. Additionally, a long period of reaction time and the sintering of produced particles are needed for hydrothermal synthesis.
According to wavelength, UV light is subdivided into UV-A and UV-B. Both of these UV radiations are significantly harmful to the skin. For example, the skin turns dark (sun-tan) when excessively exposed to UV-A, and red (sun-burn) when to UV-B. Thus, the development of organic or inorganic UV light shielding materials has been directed to protection against both UV-A and UV-B.
Among the organic UV-light shielding materials developed so far, ones with effective UV-A blocking performance are rare. In addition, organic UV-light shielding materials are found to cause skin irritation which is difficult to eliminate. Thus, as such, development research is being focused on inorganic UV light-shielding materials, e.g., titanium dioxide, zinc oxide, etc. However, the inorganic UV-light shielding materials may decompose the organic materials used together therewith, and/or the lipid components on the surface of the skin, because of their catalytic activities. It is thus important to lower the catalytic activities as well as to increase UV-light shielding effects. The smaller in particle size they are, the more effective in UV light shielding ability the inorganic materials are. Further, the inorganic particles of 60 nm or less are transparent as well as showing excellent UV light shielding effects. However, catalytic activities also tend to increase as the size decreases. Accordingly, there is a need for controlling these counter-functional properties.
In this regard, EP 1,055,642 A2 discloses a metal oxide doped ceria which has an excellent UV light shielding effect and transparency, and whose catalytic activity is low. However, much must be done to improve properties of the doped ceria.
Ceria, zirconia and ceria composites have been used in the oxygen storage application. Three-way catalysts for automobile exhausts have excellent conversion efficiency for carbon monoxide (CO), hydrocarbons, and nitrogen oxides (NOx) in a narrow air/fuel ratio range of around 14.6, but the efficiency sharply decreases outside of the range. Due to ready conversion from Ce(III) to Ce(IV) and vice versa, ceria can be used to store oxygen in the fuel lean operation and to release oxygen in the fuel rich operation.
Fuel lean: Ce(III)2O3+1/2O2→Ce(IV)O2 
Fuel rich: Ce(IV)O2→Ce(III)2O3+1/2O2 
Because of its ability to prevent the problem that the conversion efficiency of three-way catalysts is greatly decreased with even a small change in air/fuel ratio, ceria has been used with three-way catalysts since early 1990s. Three-way catalysts for automobile exhausts are necessarily exposed to high temperatures. In general, the heat resistance of ceria is not high. Accordingly, when exposed to high temperature, ceria undergo the pore filling and the sintering of crystallites, which results in a great loss of surface area, a great increase of crystallite size, and a reduction of oxygen storage capacity and oxygen mobility.
Various attempts have been made to overcame these problems.
As it was found that, when being mixed, especially when forming a solid solution with zirconia, the ceria are improved in thermal resistance as well as in oxygen storage and release capacity, the zirconia-ceria mixed oxides have been applied to three-way catalysts for automobile exhaust. It is also known that the addition of third elements can bring about an improvement in the thermal resistance and oxygen storage capacity of ceria/zirconia composites and that preparation processes and/or compositions have a great effect on the performance of the resulting composites.
Ceria/zirconia composites can be simply prepared by co-precipitation of their precursors and calcinations of co-precipitates at 500-900° C. (Japanese Patent Laid-Open No. Hei. 4-55315). Upon co-precipitation, the solubility of cerium precursors is quite different from that of zirconium precursors according to pH, so that the co-precipitates have non-homogeneous compositions. Further, the solid solution is not formed in the co-precipitation stage, and thus a calcination process should be performed.
Following the impregnation of an aqueous zirconium solution into ceria, the calcination of the impregnant at 700-1,200° C. produces ceria/zirconia composites (Japanese Patent Laid-Open No. Hei. 4-284847). However, these composites have coarse primary particle sizes and nonhomogeneous compositions.
Both of the above-mentioned processes result in unsatisfactory solid solubilities of about 40% and 20%, respectively, requiring a calcination process at high temperatures and preferably at around 1,600° C. to obtain a sufficient solid solubility of about 100%. However, the solid solutions are not suitable as oxygen storage materials for automobile exhausts because they are large in crystallite size, e.g., 1,000 nm or larger, and small in specific surface area, e.g., 1 m2/g or less.
Korean Patent No. 0313409 discloses a composition based on ceria and zirconia with a cerium/zirconium atom ratio of 1 or higher, optionally added with yttrium, scandium or rare earth metal (elements of atomic Nos 57 to 71) oxides, which have a specific surface area of 35 m2/g or larger after calcination for six hours at 900° C. This composition is prepared by mixing and heating cerium compounds, zirconium compounds and if necessary, yttrium, scandium or rare earth metal compounds in liquid media and calcining the precipitates thus produced. As mentioned above, the calcination, which is conducted to improve the crystallinity of the precipitates, may cause an excessive increase of size of crystallites.
U.S. Pat. No. 5,908,800 describes a process for preparing a mixed cerium and zirconia-based composition, which is of a single cubic phase, comprising the steps of preparing a liquid mixture containing cerium(III) and zirconium, bringing the liquid mixture into contact with carbonate or bicarbonate to form a reactive medium exhibiting a neutral or basic pH during the reaction, collecting precipitates containing cerium carbonate, and calcining the precipitates. After the calcination at 800° C. for six hours, the composition is found to have a specific surface area of 20 m2/g or higher, which remains insufficient for practical application.
Japanese Pat. No. 3341973 discloses a preparation technique of oxide solid solution particles which are improved in oxygen storage capacity and contain crystallites with an average diameter of 100 nm or lower and preferably 12 nm or lower and a specific surface area of 20 m2/g or larger and preferably 50 m2/g or larger by increasing to 70% or higher and preferably to 90% or higher the solid solubility of a mixture containing ceria, zirconia and if necessary, at least one selected from among alkaline earth metal elements and rare earth metal elements except for cerium. The solid solution particles are prepared by a first process of adding surfactant and an alkaline material or hydrogen peroxide to an aqueous solution of cerium compound and zirconium compound to give precipitates and a second process of heating the precipitates at 250° C. to facilitate the dissolution of zirconia to ceria to yield oxide solid solution particles. In Example 19, solid solution particles are described to have a specific surface area of 80 m2/g and an average crystallite diameter of 6 nm The particles, when thermally treated at 300-1,200° C. for five hours, are reduced from 75 m2/g to 5 m2/g in specific surface area and increased from 6 nm to 22 nm in average crystallite diameter. Based on this fact, the particles are believed to still be insufficient in thermal resistance.